A group of scientists have concluded, after studying three decades of data from hundreds of weather stations, that fog is decreasing throughout Europe. The reason, apparently, is cleaner air, “a fall in emissions of sulfur dioxide, a gas associated with burning coal and oil,” according to Bloomberg. Fog develops “when water droplets are suspended around particles in the air.” The particles are present because of pollution.

The decline in fog “may also have contributed to warming of the continent in recent years,” the scientists said, but fog’s influence on air temperature “will wane in the future … as the air becomes cleaner,” reported Bloomberg.

London’s fog dates back “at least 700 years” when a certain coal was banned in efforts to improve air quality. But industrialization in the 1800s caused another heavily polluted fog, eventually culminating with the “Great Smog” of 1952, which caused nearly 4,000 heart and lung disease-related deaths, reported Bloomberg.

Other deep fogs occur far beyond London, and have different causes. In Los Angeles, the “enormous driving population … and scarce options for public transportation” result in smog. Like London, California was a trailblazer, developing the “first statewide air-quality standards” in the United States.

And in Delhi, India, air and rail traffic were disrupted in December by a “heavy fog blanket” caused by a “high pollution level and winter agriculture in the northern plains,” according to the Deccan Herald. Over the past two decades in Delhi, meteorologists have seen a drastic increase in fog. Furthermore, the two main causes, “increasing pollution level and high moisture content … are still present in the atmosphere.”

The Smog of December 1952 occurred during an unusually cold winter in which there were many coal fires heating people's homes. Other pollutants were brought on a wind that died out, "trapping" the fog in the London area. The low visibility halted air, road and train travel.

"Theatres had to be suspended when fog in the auditorium made conditions intolerable. But, most importantly the smoke laden fog that shrouded the capital brought the premature death of an estimated 12,000 people and illness to many others," according to a site on air pollution created by a group of medical students at Edinburgh University.